Proximity detectors for detecting ferromagnetic objects are known. In proximity detectors, the magnetic field associated with the ferromagnetic is detected by a magnetic field-to-voltage transducer (also referred to herein as a magnetic field sensing element), such as a Hall effect element or a magnetoresistance element, which provides a signal (i.e., a magnetic field signal) proportional to a detected magnetic field.
Some proximity detectors merely provide an output signal representative of the proximity of the ferromagnetic object. However, other proximity detectors, i.e., rotation detectors, provide an output signal representative of the approach and retreat of each tooth of a rotating ferromagnetic gear or of each segment of a segmented ring magnet having segments with alternating polarity. The proximity detector (rotation detector) processes the magnetic field signal to generate an output signal that changes state each time the magnetic field signal either reaches a peak (positive or negative peak) or crosses a threshold level. Therefore, the output signal, which has an edge rate or period, is indicative of a rotation and a speed of rotation of the ferromagnetic gear or of the ring magnet.
In one type of proximity detector (rotation detector), sometimes referred to as a peak-to-peak percentage detector (or threshold detector), a threshold level is equal to a percentage of the peak-to-peak magnetic field signal. For this type of proximity detector (rotation detector), the output signal changes state when the magnetic field signal crosses the threshold level. One such peak-to-peak percentage detector is described in U.S. Pat. No. 5,917,320 entitled “Detection of Passing Magnetic Articles While Periodically Adapting Detection Threshold” assigned to the assignee of the present invention and incorporated herein by reference.
In another type of proximity detector (rotation detector), sometimes referred to as a slope-activated detector or as a peak-referenced detector (or peak detector), threshold levels differ from the positive and negative peaks (i.e., the peaks and valleys) of the magnetic field signal by a predetermined amount. Thus, in this type of proximity detector (rotation detector), the output signal changes state when the magnetic field signal departs from a peak and/or valley by the predetermined amount. One such slope-activated detector is described in U.S. Pat. No. 6,091,239 entitled “Detection Of Passing Magnetic Articles With a Peak Referenced Threshold Detector,” which is assigned to the assignee of the present invention and incorporated herein by reference. Another such peak-referenced proximity detector is described in U.S. Pat. No. 6,693,419, entitled “Proximity Detector,” which is assigned to the assignee of the present invention and incorporated herein by reference. Another such peak-referenced proximity detector is described in U.S. Pat. No. 7,199,579, entitled “Proximity Detector,” which is assigned to the assignee of the present invention and incorporated herein by reference.
It should be understood that, because the above-described peak-to-peak percentage detector (threshold detector) and the above-described peak-referenced detector (peak detector) both have circuitry that can identify the positive and negative peaks of a magnetic field signal, the peak-to-peak percentage detector and the peak-referenced detector both include a peak detector circuit adapted to detect a positive peak and a negative peak of the magnetic field signal. Each, however, uses the detected peaks in different ways.
In order to accurately detect the positive and negative peaks of a magnetic field signal, some proximity detectors, i.e., rotation detectors, are capable of tracking at least part of the magnetic field signal. To this end, typically, one or more digital-to-analog converters (DACs) can be used to generate a tracking signal, which tracks the magnetic field signal. For example, in the above-referenced U.S. Pat. Nos. 5,917,320 and 6,091,239, two DACs are used, one (PDAC) to detect the positive peaks of the magnetic field signal and the other (NDAC) to detect the negative peaks of the magnetic field signal.
Some proximity detectors are configured to be able to identify a vibration, for example, either a rotational vibration or a linear vibration of a gear or ring magnet, which vibration can generate signals from a magnetic field sensing element (magnetic field signals) that might appear similar to signals that would be generated during a rotation of the gear or ring magnet in normal operation. Proximity detectors having vibration processors that can detect a vibration are described in U.S. patent application Ser. No. 10/942,577, filed Sep. 16, 2004, entitled “Methods and Apparatus for Vibration Detection,” and in U.S. patent application Ser. No. 11/085,648, filed Mar. 21, 2005, entitled “Proximity Detector Having a Sequential Flow State Machine,” both of which are assigned to the assignee of the present invention and incorporated herein by reference.
As described above, an output signal generated by a conventional proximity detector used to detect a rotation of a ferromagnetic object (e.g., a ring magnet or a ferromagnetic gear) can have a format indicative of the rotation and of the speed of rotation of the ferromagnetic object or ring magnet. Namely, the conventional proximity detector can generate the output signal as a two-state binary signal having a frequency indicative of the speed of rotation. When the ferromagnetic object is not rotating, the conventional proximity detector can generate an inactive output signal. However the output signal generated by most conventional proximity detectors is not indicative of a direction of rotation of the ferromagnetic object.